Crystallization rate as a function of temperature is often critical when injection molding semicrystalline engineering thermoplastics. Crystallization rate as a function of temperature controls the optimum mold temperature and cycle time of the process. It is desirable to operate at mold temperatures less than 110.degree. C. because this allows for the use of traditional water heated, as opposed to oil heated, molds. These low mold temperatures also allow the process to operate at an optimum crystallization rate, which in turn translates into shorter cycle times and improved economies.
The use of a plasticizer is well known to the art to enhance crystallization rate. A plasticizer typically decreases the melt viscosity and depresses the glass transition temperature of the thermoplastic, which in turn increases crystallization rate at a lower temperature. Common plasticizers for polyester engineering plastics are low molecular weight organic esters such as neopentylglycoldibenzoate (Benzoflex S312) and dipropyleneglycoldienzoate (Benzoflex 9-88). Alternate plasticizers are poly(ether esters) such as copolyesters of poly(butylene terephthalate) and poly(tetramethylene glycol) (Hytrel).
Another key requirement when processing polyesters is drying. It is important to minimize or eliminate moisture from a polyester prior to melt processing, otherwise hydrolytic degradation occurs resulting in a diminished molecular weight and unacceptable mechanical properties. Furthermore, because drying results in an increased processing cost it is important to minimize the drying time required. Thus it is an advantage to dry at higher temperatures as this reduces the time necessary. However, many of the plasticizers and flow aids used in the art are volatile under drying conditions. Volatile emissions are undesirable because they contaminate the dryers and increase cleaning and maintenance costs.
Low molecular weight organic esters are known plasticizers for polyesters, but they tend to be volatile in the dryers, which can be remedied only by lowering drying temperatures and increasing drying time. Increasing the molecular weight of organic esters is known to reduce volatility during drying, however it is taught that this approach is not effective because the higher molecular organic esters are no longer plasticizers.
Low volatility has previously been considered advantageous because it allows higher temperatures and shorter times for melt processing. Poly(alkylene ether)s have been reported to be such non-volatile processing aids for polyesters. Suprisingly, however, many of these non-volatile poly(alkylene ether)s are volatile during the drying process over the relatively long times required for drying. This volatility results in contaminated dryers and loss of productivity.